Sooper Looper – Is a live looping sampler.  Its features and operation were inspired by the Gibson Echoplex Digital Pro.  SooperLooper allows you to record audio and immediately begin playing it as a loop.  You can overdub on top of this loop, or record additional independent loops.  Some other features allow you to sync loops to tempo, control feedback, multiply and divide loop length, insert audio into an existing loop, substitute audio in a loop while listening to existing material and many others.  All features can be controlled via MIDI (requires a program such as Midi Patchbay described below).  Sooper Looper requires Jack – discussed below.  Available for Mac OS X and Linux.

UPDATE – I recently published a video tutorial on Getting Started with SooperLooper.

Pure Data (also called Pd) – is a graphical programming environment similar to Max. Pd was developed by Miller Puckette, and provides the main features of Max and IRCAM’s “Faster Than Sound” (FTS).  Pd integrates audio synthesis, signal processing, video processing, and 3-D graphics in a real-time software environment.  There is a tutorial with sample Pd patches that can be downloaded at http://www.pd-tutorial.com.  Pure Data is available for Mac OS X, Windows, and Linux

Jack is a router for audio data. It routes audio between an audio interface and applications. Jack allows audio from any number of applications to be sent to the audio interface.  It also allows applications to send and receive audio among one another.  Be sure to read the excellent documentation before installing and using Jack.  The documentation will help you avoid potential problems.  Available for Mac OS X and Linux.

MIDI Patchbay – is like Jack for MIDI data. MIDI Patchbay routes midi data to and from a MIDI interface and software applications. In addition it allows filtering of MIDI data by note or channel, remapping of channel data, and transposition of note data. For Mac OS X.

These have been great programs to use alone or with other commercially available applications.  These applications provide exciting and powerful possibilites not generally available – and when these programs are used together, their power is expanded dramatically.

If you would like to find out about additional free software for music and sound check out Free Software For Music Creation and Experimental Computer Music and Electronic Music Creation.